Earth relative navigation can be achieved with the aid of a satellite navigation system. Several satellite navigation systems exist or are in the process of being deployed including the Global Positioning System (GPS), the Global Navigation Satellite System (GLONASS) system, the Beidou navigation system, and the Galileo positioning system.
These satellite navigation systems include a plurality of orbiting satellites that transmit signals that can be received and processed by a satellite navigation receiver in order to determine a relative earth location. Often satellites in a satellite navigation system transmit multiple signals on different frequencies. These signals on different frequencies are often referred to by the letter “L” followed by a number to differentiate different frequencies. For example, a first signal on a first carrier frequency from a first satellite is often referred to as an “L1” signal from that satellite and a second signal on a second carrier frequency from the first satellite is often referred to as an “L2” signal.
Conventional navigation systems have typically processed L1 iono corrected pseudorange and L1 delta range signals. Delta range is the difference in accumulated phase from one GPS measurement epoch to the next. Accumulated carrier phase data is often used for relative kinematic GPS solutions but is not often used for absolute earth relative navigation.
An Extended Kalman filter (EKF) can be used to obtain navigation information from the satellite signals. A baseline EKF can process three sets of measurement data from each satellite. These sets of measurement data can include L1 pseudorange, L1 delta range, and L2 delta range. The baseline EKF accounts for errors in the measurement data through the use of a combination of measurement error states with process noise and measurement observation noise. For example, the baseline EKF can maintain 10 pseudorange bias states, 10 delta range bias states, and 2 receiver clock states to model GPS measurement errors. Ten states were chosen because the baseline EKF was designed for a receiver that had 12 channels. Ten channels are used to maintain tracking and two channels are allocated to iono compensation and satellite management.
The baseline EKF can allocate measurement errors according to the following: space and control segment errors and troposphere errors are allocated to pseudorange bias states; residual ionospheric delay (after computation) errors and multipath errors are allocated to pseudorange measurement noise; receiver noise and tracking errors are allocated to pseudorange measurement noise and delta range measurement noise; residual ionospheric delay rate errors are allocated to delta range bias states; space and control segment rate errors are allocated to delta range process noise; and receiver clock errors are allocated to clock bias state and clock frequency state.